Thermosetting resins are commonly used in adhesive formulations due to the outstanding performance properties which can be achieved by forming a fully crosslinked (i.e., thermoset), three-dimensional network. These properties include cohesive bond strength, resistance to thermal and oxidative damage, and low moisture uptake. As a result, common thermosetting resins such as epoxy resins, bismaleimide resins, and cyanate ester resins have been employed extensively in applications ranging from structural adhesives (e.g., construction and aerospace applications) to microelectronics (e.g., die-attach and underfill applications).
Although thermosetting resins have been used successfully as adhesives in a variety of industries, a property inherent to all thermosetting resins which negatively impacts adhesion performance is shrinkage upon cure. This phenomenon is attributed to the formation of a three-dimensional, covalently crosslinked network during cure, which reduces intermolecular distances between the monomers used to form the crosslinked network. For example, before cure, the molecules which comprise the resin are separated by their characteristic van der Waal's radii. Upon cure, these intermolecular distances are reduced due to the formation of covalent bonds between monomers which produces the desired highly crosslinked thermoset material. This reduction of intermolecular distances creates internal stress throughout the thermoset network, which is manifested by reduced adhesion of the thermoset material to both the substrate and the object attached thereto.
It is well established that thermosetting monomers which cure via ring-opening chemistry (e.g., epoxies and benzoxazines) or ring-formation chemistry (e.g., cyanate esters) have reduced cure shrinkage. Ring-opening cure chemistry is advantageous since this physical transformation helps diminish volumetric shrinkage on cure. Similarly, ring-formation cure chemistry acts to reduce shrinkage due to the slight expansion which occurs upon ring-formation (much like that which occurs when water freezes to form ice crystals). Accordingly, epoxies, benzoxazines and cyanate esters all have excellent adhesive properties—presumably due to their diminished cure shrinkage. However, virtually all free-radically polymerized monomers do not participate in ring-opening or ring-forming reactions and therefore often exhibit severe cure shrinkage.
Strategies have been developed to address the problem of shrinkage upon cure. Common approaches include the addition of an inorganic filler to the adhesive formulation, and/or the use of a higher molecular weight thermosetting material. However, both of these strategies undesirably increase the viscosity of the final adhesive formulation. Accordingly, there remains a need for low shrinkage upon cure thermosetting resin compositions which do not detract from the properties of the adhesive formulation, in either its cured or uncured state, or its cure profile.